The trans‐Saharan spring migration of Yellow wagtails (Motacilla flava)

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Abstract
By considering the weights of the heaviest members of a population of Yellow wagtails about to emigrate from northern Nigeria, estimates of the mean weight of each sex at take‐off are derived. These are used to predict changes in weight and flight speed during a non‐stop trans‐Saharan flight. For most birds the duration of this flight will be 60–70 h, enabling them just to attain the northern side of the desert. Some birds may resort to partial dehydration to achieve this, but extensive dehydration is not apparent for most birds caught towards the end of the flight. It is apparently essential that favourable winds occur during the flight; even slight head winds are likely to produce heavy mortalities. Because Yellow wagtails depart on a trans‐Saharan crossing with just enough reserves to achieve this, adverse winds, difficult conditions immediately north or south of the desert, or any avoidable extension of the flight are all likely to impose considerable selection pressures. Therefore, probably all Yellow wagtails move gradually northward in Nigeria, to the southern margins of the desert, where they may still obtain adequate food, and there await favourable meteorological conditions before embarking.